1. Field of the Invention
This invention relates to a continuous process of smelting metallic lead directly from lead- and sulfur-containing materials in an elongated horizontal reactor, wherein a molten bath consisting of a slag phase and a lead phase is maintained in the reactor, the slag phase and the lead phase are countercurrently conducted through the reactor, the gas atmosphere is conducted countercurrently to the slag phase through the reactor, oxygen is blown into the molten bath from below at controlled rates in the oxidizing zone, which is disposed on the side where the lead is tapped, lead- and sulfur-containing material is charged at controlled rates onto the molten bath, reducing agent is introduced into the molten bath in the reducing zone, which is disposed on the side where the slag is tapped, additional heat is supplied to the gas space in the reducing zone, such an oxidation potential is maintained in the oxidizing zone that the charge is smelted in a thermally self-sufficient process to form metallic lead and a slag which contains lead oxide, and the rate of the reducing agent and the temperature in the reducing zone are so controlled that a low-lead slag is formed.
2. Discussion of Prior Art
German Offenlegungsschrift No. 28 07 964 discloses a continuous process of converting lead sulfide concentrates into a liquid lead phase and a slag phase under a gas atmosphere having SO.sub.2 -containing zones in an elongated horizontal reactor. In that known process, lead sulfide concentrates and fluxes are charged onto the molten bath. The lead phase and a low-lead slag phase are discharged at mutually opposite ends of the reactor. The phases flow countercurrently to each other in substantially continuous layer-forming streams to the outlet ends. At least part of the oxygen is blown into the molten bath from below through a plurality of mutually independently controlled nozzles, which are distributed over the length of the oxidizing zone of the reactor. The solid charge is charged into the reactor in several stages through a plurality of mutually independently controlled feeders, which are distributed over a substantial length of the reactor.
The locations and rates at which oxygen and solids are fed are so selected that the gradient of the oxygen activity in the molten bath has at the end where lead is tapped a maximum for the production of lead and from said maximum decreases progressively to a minimum for the production of low-lead slag phase, which minimum is obtained at the end where said slag phase is tapped.
Gaseous and/or liquid protective fluids are blown into the molten bath at controlled rates together with the oxygen and serve to protect the nozzles and the surrounding lining and to assist the control of the process temperature. The rates at which gases are blown into the molten bath are so controlled that the resulting turbulence is sufficient for a good mass transfer but will not substantially disturb the flow of the phases in layers and the gradient of the oxygen activity. The gas atmosphere in the reactor is conducted countercurrently to the direction of flow of the slag phase. The exhaust gas is withdrawn from the reactor at the end where the lead phase is tapped. To produce a low-lead slag, reducing agents are introduced into the reducing zone and additional heat is supplied into the gas space in said zone so that the heat to be absorbed in reaction is supplied and the slag is heated in the reducing zone. Stilling zones in which no gases are blown into the molten bath may be provided between the oxidizing and reducing zones and also before the oxidizing zone and behind the reducing zone.
The temperature of the molten bath in the oxidizing and reducing zones should be kept as low as possible so that an attack of overheated slag on the brickwork will be avoided as well as the need for the otherwise required cooling of the brickwork at higher temperatures, also a strong evaporation of metals or metal compounds and an unnecessary heating of the lead phase. But low processing temperatures involve a risk of an undercooling of the molten bath during fluctuations in operation.
German Pat. No. 23 20 548 discloses a direct lead-melting process wherein a mixture of fine-grained lead sulfide and oxygen impinges on a molten bath from above with ignition and formation of a flame. A considerable part of the oxidation is already effected in the furnace atmosphere. The flame temperature is above 1300.degree. C. and the temperature of the molten bath between 1100.degree. and 1300.degree. C. in the oxidizing zone. The slag phase and the furnace atmospheres are countercurrently conducted through the furnace. A slag containing at least 35% lead as lead oxide is tapped from the furnace and is reduced in a separate reducing furnace. 98% to 120% of the quantity of oxygen which would be stoichiometrically required for a complete conversion of the lead sulfide to metallic lead are needed to produce the lead phase. To control the furnace temperature, about 120% oxygen can be added during short periods to effect an increased transfer of lead oxide to the slag. But that temperature control cannot be adopted in the above-described process carried out in a reactor which includes oxidizing and reducing zones and from which a low-lead slag is tapped. Besides, that temperature control will not avoid the disadvantages involved in high temperatures of the molten bath and in an overheated slag.